Internal combustion engine exhaust emissions, and especially diesel engine exhaust emissions, have recently come under scrutiny with the advent of stricter regulations, both in the U.S. and abroad. While diesel engines are known to be more economical to run than spark-ignited engines, diesel engines inherently suffer disadvantages in the area of emissions. For example, in a diesel engine, fuel is injected during the compression stroke, as opposed to during the intake stroke in a spark-ignited engine. As a result, a diesel engine has less time to thoroughly mix the air and fuel before ignition occurs. The consequence is that diesel engine exhaust contains incompletely burned fuel known as particulate matter.
It is known to use particulate traps which physically trap the particulates. Such traps are expected to be in widespread use on all diesel market segments beginning in 2007. However, diesel particulate traps (DPT), both catalyzed and non-catalyzed, tend to load up with accumulated soot and therefore must be repeatedly regenerated by catalytically oxidizing the trapped particulates.
Because a diesel engine runs relatively lean (approximately 60:1 air/fuel ratio at idle), nitrogen oxides (NOx) are readily developed in diesel exhaust. It is also known to use a catalytic trap (Lean NOx Trap, or LNT) to remove nitrogen oxides from a diesel exhaust stream. Typically, oxides of nitrogen are trapped by reaction with barium cation in an LNT. Because the capacity of such a trap is finite, an LNT also requires periodic regeneration.
A currently challenging durability issue in the DPT art is cracking or melting of a substrate due to large temperature excursions within the bed of the filter. These temperature excursions are caused by the exothermic reaction of carbon and oxygen when the soot loading exceeds a critical level (approximately 5 grams per liter of cordierite substrate, and approximately 10 grams per liter of silicon carbide substrate) and the flow rate of exhaust through the DPT is reduced by idle or low-load engine operating conditions. Under these conditions, the exhaust contains a high percentage of oxygen (18% or more), thus fueling a very rapid combustion of the soot, but at a low total flow rate, thus reducing convective cooling of the hot substrate. This combination of events (rapid combustion and low cooling) can result in excessive filter temperature and/or temperature gradients, resulting in substrate failure.
What is needed in the art is a means for regulating the temperature of combustion of soot in a DPT during regeneration thereof to prevent damage to the DPT substrate.
It is a principal object of the present invention to prevent damage to a DPT substrate by overheating during regeneration thereof.